1. Field of the Invention
The present invention relates to a cooling plate structure, incorporated into a fuel cell stack, for eliminating waste heat generated by a fuel cell, using water as a cooling medium.
2. Description of the Related Art
FIG. 5 illustrates a conventional fuel cell stack equipped with a conventional cooling plate. In FIG. 5, a single-cell 1 is composed of a matrix layer 11 for holding an electrolyte, a fuel pole 12, an oxidizer pole 13, electrode bases 14 and 15 each formed with ribs, and separators 16. A plurality of the thus constructed single-cells 1 are laminated to form a cell stack 2. Water-cooling type cooling plates 3 are disposed in the cell stack at several-cell intervals. Each cooling plate 3 is formed as an assembled body comprising: a carbon cooling substrate 4, the thermal expansion coefficient of which is substantially equal to those of the electrode bases 14 and 15 formed with the ribs and of the separator 16; and metallic cooling pipes 5 arranged in parallel and embedded in a layer of the cooling substrate 4. The cooling pipes 5 are connected as a bank to a header pipe 6 and further connected to an external cooling water supply line (not shown).
Embedding the cooling pipes 5 into the cooling substrate 4 requires a pipe arrangement wherein the cooling pipes 5 are accommodated in a plurality of pipe grooves sandwiched between adjoining parallel surfaces of two cooling substrates. An alternate arrangement is a plurality of U-shaped pipe grooves chased in the surface of the single cooling substrate 4, with such pipe grooves provided with a cover formed of the same carbon material as the substrate after placing the respective cooling pipes 5 therein. Note that the cooling plate 3 is used for removing the waste heat generated by the cell by causing the cooling pipes 5 to admit a cooling medium, for example water, supplied from the outside during steady-state operation of the fuel cell. The fuel cell cooling plate 3 also serves an additional function of increasing the fuel cell body temperature from a low temperature to a starting temperature during fuel cell activation, by causing hot water to flow through the cooling pipes 5.
There are, however, some drawbacks to the conventional apparatus. It is quite difficult to completely set the entire peripheral surfaces of the cooling pipes 5 in total circumferential physical contact with the pipe groove surfaces of the cooling substrate 4 due to dimensional tolerances of the cooling pipes and processing inaccuracies in forming the pipe grooves in the cooling substrate 4. It is inevitable that there will exist slight air gaps between the outer pipe circumferences and surfaces of the grooves. A thermal resistance associated with the air gaps is substantially greater than that of the cooling substrate 4 and the cooling pipes 5. Hence, if even a small number of such air gaps exist between the cooling pipes 5 and the pipes grooves of the cooling substrate 4, heat transfer declines remarkably.
A method has been developed to attempt to cope with this problem, in which the apparatus is designed with the residual air gaps filled with a charging substance. This substance is produced by mixing a graphite-loaded ceramic material, having a high heat transfer coefficient, with thermosetting resin after the cooling pipes 5 have been placed in the pipe grooves of the cooling substrate 4. The charging substance reduces the thermal resistance existing between the cooling substrate 4 and the cooling pipes 5.
The above-mentioned structure for a cooling plate has, however, also produced unsatisfactory results when the fuel cell is operated. For example, in the above-described construction, the thermosetting resin is disposed between the cooling substrate and the cooling pipes in order to bond them together. A high degree of heat transfer is achieved during the early stages of operation. However, because of a difference in the thermal expansion coefficients between the metallic cooling pipes 5 and the carbon cooling substrate 4, the thermal resistance between the cooling pipes 5 and the solid charging substance increases with exposure to numerous heat cycles. In particular, operations involving repetitive starting and stopping of the fuel cell result in exfoliation of the charging substance during long stretches of use. Further, unnecessary thermal stress acts on the cooling substrate 4, attributable to the difference in thermal expansion coefficients between the cooling substrate 4 and the cooling pipes 5. In the worst case, this can result in cracks in the carbon cooling substrate 4.